In a medical ultrasound imaging system, ultrasound pulses are usually transmitted into a body using a transmitting circuit. The ultrasound waves are reflected by interfaces of tissue within the body. Visible ultrasound images of the tissue within the body may be obtained by receiving and processing echoes which carry characteristic information about the tissue within the body.
During ultrasound imaging, weak boundaries and small vessels sometimes may be vaguely imaged or may not be imaged at all because of reverberation and the limitation of resolution. During contrast imaging, there may be a big difference between acoustic impedances of a contrast agent and the surrounding tissues, which may change absorption, reflection, scattering and refraction of acoustic waves within the tissues, thereby enhancing echo signals and improving contrast resolution. In addition, microbubbles of the contrast agent have significant nonlinear characteristics. When being excited by ultrasound pulses, the degrees of stretch and expansion of the microbubbles are different, such that the ultrasound echoes reflected by them contain not only linear components corresponding to the original ultrasound pulses, but also nonlinear components. In the ultrasound echoes reflected by body tissues containing the contrast agent, the linear components contain not only the linear components from the tissue, but also the linear components from the contrast agent. Ultrasound images obtained by detecting and processing fundamental linear components have low contrast resolution and cannot clearly present perfusion of the contrast agent in microangium and tissues, which may affect clinical diagnosis. Therefore, detecting the nonlinear components in the ultrasound echoes during ultrasound contrast imaging may be necessary.